The current studies have shown that, after lesions in one of the four major spinal systems (ipsilateral or contralateral dorsal roots, ipsilateral or contralateral descending systems) initial depression of all motor behavior is followed by selective return of certain movements and reflex patterns. The recovery is not mediated equally by all the remaining systems; rather, only one predominates, and is specific to the lesion. The system responsible for the recovery undergoes collateral sprouting during the recovery period. Preliminary work shows that the other remaining systems show no sign of having sprouted. Therefore, a hierarchical or competitive control of recovery and of collateral sprouting by the several remaining systems has been proposed. Several studies are proposed in order to determine the behavior and anatomical correlates of this concept. 1) Clarke's nucleus receives sprouted fibers from a spared dorsal root after partial deafferentation and from descending systems after total deafferentation. A fine structural study of partially and completely deafferented Clarke's nucleus in acute and chronic cats has been begun. 2) A radioautographic study comparing the projection of the spared root and the normal contralateral one has been initiated. 3) A behavioral study to localize the source of recovery in totally and partially deafferented cats is in progress.